Electroluminescent lamp manufacture



Dec. 28, 1965 w. LONGFELLOW 3,226,272

ELECTROLUMINES CENT LAMP MANUFACTURE Filed Sept. 13, 1961ITlVfiBTWTCDT". fievber t W. Lo cgFeLLow mfzr United States Patent3,226,272 ELECTROLUMINESCENT LAMP MANUFACTURE Herbert W. Longfellow,tCincinnati, Ohio, assignor to General Electric Company, a corporationof New York Filed Sept. 13, 1961, Scr. No. 137,924 7 Claims. (Cl.156-67) This invention relates in general to electroluminescent devicesand to a light-transmitting electrically conductive plastic filmmaterial therefor, and is especially concerned with a new and improvedmethod for making a flexible type laminated electroluminescent cell. Thepresent application is a continuation-in-part of my prior copendingapplication Serial No. 126,103, filed July 24, 1961, and assigned to thesame assignee as the present invention.

As it is presently known in the art, an electroluminescent cellgenerally comprises a phosphor layer sandwiched between two conductinglayers, one of which is transparent or is, at least, light transmitting.A well known form of such an electroluminescent cell is described in US.Patent 2,945,976 issued to Fridrich et al., and assigned to the sameassignee as the present invention. The lamp therein disclosed comprisesa flexible laminated assembly of electrically active layers encased in athin envelope of thermoplastic material which has been evacuated andheat sealed around its edges. The electrically active elements containedin such a lamp generally consist of a flexible conductive layer composedof aluminum foil coated with a layer of a high dielectric constantmaterial which, in turn, is overcoated with a layer of anelectroluminescent phosphor, and finally overlayed with alight-transmitting conductive sheet. In addition, a thin flexible filmof a suitable thermoplastic material such as low density polyethylene ornylon 6 may preferably be placed over the light-transmitting conductingsheet which thermoplastic film, following the laminating step, firmlyholds the conducting sheet in place and cements it to both theunderlying phosphor layer and the overlying layer of the encasingthermoplastic envelope.

In a structure such as described in the aforesaid Fridrich et al.patent, the aluminum foil and the electrically conductive sheetconstitute the electrodes of the assembled electroluminescent unit.Accordingly, the application of an alternating potential across theseelectrodes results in the excitation of the phosphor with a consequentemission of light through the respective light-transmitting layers ofthe lamp assembly.

The light-transmitting conductive sheet employed in the production of anelectroluminescent cell such as that described above heretofore has beengenerally comprised of conducting glass paper such as, for example, thecommercially available micro-fiber glass paper around .002 inch thick.Conductivity is imparted to such paper by dipping the paper in asolution of a suitable metal salt, and subsequently drying and bakingthe paper at elevated temperatures to provide a conductive coating onthe surface portions of the constituent glass fibers. A suitablesolution employed for rendering such glass paper conductive comprisesindium basic trifluoroacetate with stannic chloride sncn dissolved in anorganic solvent such as ethylene glycol monoethyl ether acetate. In thisregard, reference may be made to US. Patent 2,849,339, Jaffe, assignedto the same assignee as the present invention for a more completedescription of the materials and processes employed in providing such aconducting glass paper.

Although the prior art methods involving the use of conducting glasspaper have resulted in the production of electroluminescent cells whichhave been entirely satis- 3,226,272 Patented Dec. 28, 1965 factory, therather fragile nature of this glass paper severely handicaps the easewith which such electroluminescent cell assemblies can be produced.Specifically, the glass paper employed, due to its extremely delicatenature, crumbles easily, and therefore requires the utmost degree ofcare while cutting, and while handling during cell assembly. Inaddition, the fragile nature of the paper imposes severe difiiculty incutting the paper into intricate shapes without simultaneouslyshattering the paper during the cutting process. Moreover, and perhapsmost importantly, it should be noted that current techniques ofproducing electroluminescent cells having an area of 36 square inches orgreater, which techniques necessarily require the dificult step ofcutting a sheet of glass paper to the appropriate dimensions, results inhigh production rejects or so-called shrinkage due apparently to theelectrical shorting of the lamps caused by the contact of the conductiveglass fibers with the underlying metallic base foil. An explanation ofthis phenomenon may be that during the subsequent pressure laminatingoperation, portions of the conducting glass paper are forced through theunderlying phosphor and insulating layers to the extent that theycontact the conductive metallic film. Whatever the cause of suchshorting may be, it remains apparent that a new and improved scheme ofpreparing electroluminescent cell assemblies which would obviate theproblems attending the handling of extremely fragile conductive glasspaper, and which would, if possible, reduce or eliminate the productionrejects in the case of larger sized electroluminescent cells due toelectrical shorting between the conducting glass paper and theunderlying base foil, would indeed be highly desirable from thestandpoint of simplicity and economy in manufacture.

It is accordingly the primary object of the present invention to providea new and improved method of making an electroluminescent cell.

Another object of the present invention is to provide a new and improvedmethod for making an electroluminescent cell of the type having aconductive glass paper electrode, which method is simpler and moreeconomical than the methods heretofore employed for the production ofsuch type cells.

A further object of the present invention is to provide a new andimproved method for making a flexible electroluminescent cell assemblyof the type employing a layer of fragile electrically conductivematerial, which method substantially eliminates the problem of handlingand cutting such fragile material without breakage thereof such as hasbeen prevalent with the methods heretofore employed in the art for themanufacture of such type cell assemblies.

Yet another object of the present invention is to provide a new andimproved method of making an electroluminescent cell assembly of thetype employing a conductive glass paper electrode, which method affordsa substantial elimination of the problem of high production rejectsheretofore encountered in such cell production as a result of theelectrical shorting of the cells due to the Contact of the conductivefibers of the conducting glass paper layer with the underlyingconducting base member of the cell assembly.

A still further object of the invention is to provide a flexiblelight-transmitting conductive plastic sheet material suitable for use asan electrode layer for an electroluminescent cell.

Briefly stated, these and other objects may be obtained in accordancewith the invention by first prelaminating the conductive glass papertogether with a suitable thermoplastic film material to form alight-transmitting electrically conductive plastic film material orlaminate of tough and flexible character which can be handled with easewithout fracturing, and then laminating a sheet of such conductiveplastic film material together with, or otherwise affixing thereto, theother component laminar elements of an electroluminescent cell, tothereby form the completed electroluminescent cell assembly.

Still other objects and advantages of the invention will be apparentfrom the following description of a species thereof and from theaccompanying drawing in which:

FIG. 1 is a pictorial view of a flexible, laminated electroluminescentcell made in accordance with the method of the present invention, thevarious constituent layers being delaminated or peeled open at onecorner to show the internal construction thereof, and

FIG. 2 is a fragmentary sectional view, on a greatly enlarged scale, ofthe light-transmitting electrically conductive plastic sheet materialcomprising my invention and constituting the light-transmittingelectrode of the electroluminescent cell.

Referring to FIG. 1, there is shown a flat rectangularelectroluminescent cell 1 made up entirely of components or layers whichhave been laminated together and sealed in a plastic outer encapsulatingenvelope in accordance with the method 'of the present invention. Thecell 1 may be energized by applying a suitable potential such as analternating voltage, for example, 120 volts, 60 cycles AC. to the copperterminals 2 and 3 projecting laterally from the edge of the plasticouter envelope which is composed of sheets 4 and 5 of plastic material.The lowermost film or lamina 4 and the uppermost lamina 5 which formrespectively the underside and the topside of the envelope in thecompleted lamp construction, consist of sheets of thermoplastic materialwhich fiow under heat and pressure, and they are heat-sealed togetheralong their margins. The materials selected for this use are preferablytough and stable in addition to exhibiting light transmitting qualitiesand high impermeability to moisture, and further they are preferablyflexible in nature. Among the materials which may be successfullyemployed in this regard are polyethylene, polytetrafiuoroethylene,polychlorotrifluoroethylene, polystyrene, methyl methacrylate,polyvinylidene chloride, polyvinyl chloride, polycarbonate materialssuch as, for example, the reaction products of diphenylcarbonate andbisphenol A, and polyethylene terephthalate. A preferred materialconsists of polychlorotrifiuoroethylene film, known as Kel-F, ofapproximately 0.005 inch thickness.

The electrically active elements of the cell 1 comprise a rectangularsheet of thin metal foil 6 coated with an insulating layer 7 (indicatedby cross-lining) of high dielectric constant material which isovercoated with a lightproducing layer 8 (indicated by speckling) of anelectroluminescent phosphor. The metal foil sheet 6 is placed over thelowermost plastic sheet 4 leaving a clear margin all around. Alight-transmitting and electrically conductive laminar sheet 9 comprisedof a layer of conductive micro-fiber glass paper is placed over thecoated side of the metal foil 6, leaving a narrow margin of the coatedside of the foil uncovered all around. The conductive glass paper layer9 and the metal foil 6 constitute the two electrodes of theelectroluminescent cell. The conductive glass fibers of the glass paperlayer 9 are preferably bound in place and cemented to the phosphor layer8 by a layer 10 of a suitable thermoplastic material in which the glassfibers are partly embedded. As shown in the drawing, the juxtaposedmargins of the bottom and top thermoplastic sheets 4 and 5 projectbeyond the edges of the metal foil 6 and are fused or sealed together toform a sealed outer envelope encapsulating the electrically activeelements of the cell.

The coated metal foil 6 may consist of dead soft annealed aluminum of,for example, around 0.002 inch thickness coated with a thin insulatinglayer 7 of barium titanate dispersed in an organic polymeric matrix andovercoated with an electroluminescent phosphor layer 8 consisting of anyknown electroluminescent phosphor such as, for example zinc sulfide-zincoxide with suitable activators such as copper, manganese, lead orsilver, likewise dispersed in an organic polymeric matrix such as thatused in connection with the insulating layer 7. Examples of suitableorganic polymeric matrixes are cellulose nitrate, polyacrylates,methacrylates, polyvinyl chloride, cellulose acetate, alkyd resins,epoxy cements, and polymers of triallyl cyanurate, to which may be addedmodifying substances or plasticizers such as camphor, dioctyl phthalate,tricresyl phosphate and similar materials. A preferred organic polymericmatrix forming a dense tough film of high dielectric constant and goodmechanical and thermal stability and consisting of cyanoethyl cellulosewith suitable plasticizers such as cyanoethyl phthalate is described andclaimed in US. Patent 2,951,865, Jaffe et al., assigned to the sameassignee as the present invention. The barium titanate layer dispersedin a cyanoethyl cellulose solution may be applied to the aluminum foilby spraying, or preferably through the use of a doctor blade, and thendrying; the phosphor layer, likewise dispersed in a cyanoethyl cellulosesolution, may then be applied over the barium titanate layer in asimilar manner.

The electroluminescent cell is energized by applying a suitablepotential between the conductive layers, that is, between aluminum foil6 and the conductive thermoplastic sheet 9. The projecting copper braidsor ribbons 2, 3 provide convenient terminals for so doing and areconnected respectively to the aluminum foil 6 and the electricallyconductive glass paper layer 9. During the laminating process, thecopper ribbons become embedded in the thermoplastic sheets 4 and 5 andare at the same time pressed against and into electrical contact withthe aluminum foil 6 and the conductive glass paper 9, as the case maybe.

The manufacture of an electroluminescent cell 1 such as described iscustomarily accomplished by stacking all the various cell components orlayers together in their proper positional relationship, and thensubjecting the stacked assembly to pressure and heat to laminate thecell components together. The laminating of the cell components may beperformed in the manner, and by the use of a hydrostatic laminatingpress such as described and claimed in Us. Patent 2,945,976, Fridrich etal. As therein described, the stacked assembly of cell components isplaced between the top and bottom platens of the hydrostatic press,beneath a conformable diaphragm separating the press platens, theconformable diaphragm being constituted of a flexible gas-impervioussheet material such as soft annealed aluminum foil or polyethyleneterephthalate film such as Mylar. Pressurized gas is admitted into theclosed chamber of the press over the diaphragm therein to exerthydrostatic pressure on the cell components, vacuum is applied under thediaphragm to remove any trapped gases or moisture from the spacetherebelow, and heat is then applied by suitable means to the stackedassembly of cell components to cause the plastic encapsulating sheets 4and 5 to soften and seal together at their margins. For a more completeunderstanding of a suitable laminating technique, reference may be madeto the aforementioned Fridrick et al. Patent 2,945,976.

According to the prior practice for making electroluminescent cells 1 asdescribed above, the glass paper layer 9 of the cell and, whereemployed, the plastic binder layer 10 also, have consisted of separatecomponent members at the time of their assembly together with the othercomponents of the cell in readiness for the laminating together thereof.Because of the highly fragile and delicate character of the glass paper9 by itself, considerable difficulty has therefore been experienced inthe handling and cutting of the glass paper without breakage thereof. Asa result, a high percentage of production rejects or shrinkage has beenencountered in the manufacture of the cell assemblies 1 by prior cellmaking techniques, thus materially increasing the manufacturing cost ofsuch devices.

In accordance with the invention, the conductive glass paper 9, prior toits lamination together with or the aflixation thereto of the othercomponents of the electroluminescent cell, is pre-laminated to asuitable thermoplastic sheet material to form a light-transmittingelectrically conductive plastic sheet material or prelaminate 11 (FIG.2) having conductive glass fibers partially embedded in one face thereofbut sufliciently exposed thereat to render the said face electricallyconductive. The electrically conductive plastic prelaminate 11 may thenbe stacked together with the other components of the cell 1, with theconductive glass fiber side of the prelaminate 11 next to the phosphorlayer 8, and the stacked assembly or lay-up then laminated togetherunder pressure and heat in the same manner such as described previouslyto thereby complete the manufacture of the electroluminescent cell 1.The glass paper 9 and the plastic sheet material may be laminatedtogether by placing the conducting glass paper 9 over the thermoplasticsheet material 10 and subjecting the stacked assembly to a pressure ofabout 400 p.s.i. and a temperature of approximately 200 C. suflicient tosoften the plastic so as to flow between the glass fibers of theconductive glass paper and cause the fibers to become partially embeddedin the face of the plastic sheet. The resulting product is alight-transmitting electrically conductive smooth surfaced plastic sheetmaterial 11 of tough and flexible character which can be handled withease and lends itself to cutting and bending without fracturing orbreaking apart. Because of the smooth surfaced character of the glassfiber embedded face of such conductive plastic sheet material 11, thelikelihood of any of the glass fibers being forced through theinsulating and phosphor layers 7 and 8 of the cell and into contact withthe underlying metal foil layer 6 thereof during the cell laminatingoperation, with resultant short circuiting of the cell and damagethereto, is greatly minimized.

In general, any thermoplastic material may be employed for the plasticsheet 10 of the conductive plastic sheet material 11 comprising myinvention. Included among the materials which have been found suitablefor such purpose, however, are nylon, cellulose acetate, celluloseacetate butyrate, polyvinyl alcohol, polyvinylpyrrolidone,polyvinylchloride, polyvinylidine chloride, copolymers ofpolyvinylchloride and polyvinylidine chloride, polyvinylacetate,polystyrene and polymers of methyl methacrylate. cell which has asubstantially improved resistance to water depreciation, it may beadvantageous to select for the plastic layer 10 a thermoplastic materialwhich exhibits hydrophilic properties, i.e., has an aflinity for water.As disclosed in copending application Serial Number 80,613 of Devol etal., filed January 4, 1961, now Patent No. 3,148,299, and assigned tothe same assignee as the present invention, polyamide condensationproducts such as nylon 66, or nylon 6 such as that known as Caplene,have been found to be particularly effective as hydrophilic materialsfor the plastic layer 10.

The pressure required to laminate together the contiguous layers ofconducting glass paper 9 and plastic sheet material 10 to form theprelaminate 11 may be applied thereto in any suitable manner, as bycompressing them between coperating flat pressure plates or pressurerolls, or by means of a gas impervious conformable diaphragm in ahydrostatic press such as described in the aforementioned Fridrich etal. Patent 2,945,976. The electrically conductive plastic prelaminatematerial according to the invention may be formed in large sheets or incontinuous roll form, from which individual sheets 11 of proper size andshape may then be cut for lamination together with the other componentelements of the electroluminescent cell 1.

In order to provide an electroluminescent Instead of laminating theconductive plastic prelaminate 11 together with the other components ofthe electroluminescent cell as described hereinabove to form thecompleted cell assembly, the prelaminate 11 may be coated with thelight-producing layer 8 of electroluminescent phosphor and thenovercoated with the insulating layer 7 of high dielectric constantmaterial, following which the second or back electrode 6 is then appliedover the insulating layer 7. This electrode layer 6 may be comprised ofsome form of electrically conductive paint or paste, or a similarconductive material which may be brushed, sprayed, rolled or silkscreened onto the insulating layer 7. Alternatively, a layer of asuitable metal such as aluminum may be applied over the insulating layer7 by evaporation under high vacuum according to techniques well known inthe art, or metal foil such as aluminum foil may be either laminated tothe insulating layer 7 or cemented thereto by conductive cement.

By utilizing the new and improved method of the present inventionwherein the conductive glass paper is initially laminated to athermoplastic sheet to form a flexible conductive plastic sheetmaterial, the problem of handling and cutting the highly fragileconductive glass paper heretofore attending the production of flexibleelectroluminescent lamps is reduced to a minimum and is, in fact, neverencountered during the course of the actual lamp assembly operation.Moreover, it has been discovered that, as a result of the new andimproved method of the present invention, the electrical shorting whichoccurs during the manufacture of electroluminescent cells by theconventional methods of the prior art has been substantially eliminated.

Although specific embodiments of the present invention have beendescribed in detail, it should be understood that the present inventionis not to be considered limited to such embodiments, but may be used inother ways without departure from the spirit of the invention and thescope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In the manufacture of an electroluminescent cell comprising a layerof electroluminescent phosphor sandwiched between a pair of electrodelayers at least one of which is light-transmitting, the method whichcomprises the steps of preliminarily forming a self-supporting, tough,flexible and light-transmitting electrically conductive sheet, which canbe readily handled and cut to size without fracturing, by applying asheet of extremely fragile electrically conductive glass paper comprisedof electrically conductive glass fibers to a face of a flexiblethermoplastic sheet and laminating the said sheets together underpressure and heat so that the said conductive glass paper is partlyembedded in the said face of the plastic sheet but sufliciently exposedthereat to render the said face electrically conductive, and thenaffixing to the said conductive face of the plastic sheet a layer ofelectroluminescent phosphor and an overlayer of electrically conductivematerial.

2. In the manufacture of an electroluminescent cell comprising a layerof electroluminescent phosphor sandwiched between a pair of electrodelayers at least one of which is light-transmitting, the method whichcomprises the steps of preliminarily forming a self-supporting, tough,flexible and light-transmitting electrically conductive sheet, which canbe readily handled and cut to size without fracturing, by applying asheet of extremely fragile electrically conductive glass paper comprisedof electrically conductive glass fibers to a face of a flexiblethermoplastic sheet and laminating the said sheets together underpressure and heat so that the said conductive glass paper is partlyembedded in the said face of the plastic sheet but sufliciently exposedthereat to render the said face electrically conductive, and thenlaminating the said conductive plastic sheet to the phosphor layer of anelectroluminescent cell sub-assembly with the said electricallyconductive face of 7 the conductive plastic sheet next to the saidphosphor layer.

3. In the manufacture of an electroluminescent cell comprising a layerof electroluminescent phosphor sandwiched between a pair of electrodelayers at least one of which is light-transmitting, the method whichcomprises the steps of preliminarily forming a self-supporting, tough,flexible and light-transmitting electrically conductive sheet, which canbe readily handled and cut to size without fracturing, by applying asheet of extremely fragile electrically conductive glass paper comprisedof electrically conductive glass fibers to a face of a flexiblethermoplastic sheet and laminating the said sheets together underpressure and heat so that the said conductive glass paper is partlyembedded in the said face of the plastic sheet but sufficiently exposedthereat to render the said face electrically conductive, coating thesaid electrically conductive face of the conductive plastic sheet with alayer of electroluminescent phosphor, and then applying an electricallyconductive layer over said phosphor layer.

4. In the manufacture of an electroluminescent cell comprising a layerof electroluminescent phosphor sandwiched between a pair of electrodelayers at least one of which is light-transmitting, the method whichcomprises the steps of preliminarily forming a self-supporting, tough,flexible and light-transmittin g electrically conductive sheet, whichcan be readily handled and cut to size without fracturing, by applying asheet of extremely fragile electrically conductive glass paper comprisedof electrically conductive glass fibers to a face of a flexiblethermoplastic sheet and laminating the said sheets together underpressure and heat so that the said conductive glass paper is partlyembedded in the said face of the plastic sheet but sufiiciently exposedthereat to render the said face electrically conductive, coating thesaid electrically conductive face of the conductive plastic sheet with alayer of electroluminescent phosphor, overcoating said phosphor layerwith an insulating layer of high-dielectric constant material, and thenapplying an electrically conductive layer over said insulating layer.

5. The method of making an electroluminescent cell as specified in claim1 wherein the said thermoplastic sheet consists of a hydrophilicthermoplastic material.

6. The method of making an electroluminescent cell as specified in claim1 wherein the said t-herornplastic sheet consists of a material selectedfrom the group consisting of polyamide condensation polymers.

7. The method of making an electroluminescent cell as specified in claim1 wherein the said thermoplastic sheet consists of nylon 6.

References ited by the Examiner UNITED STATES PATENTS 2,511,887 6/1950Vinal. 2,699,415 1/1955 Nachtman. 2,781,287 2/1957 Gustus 154-462,827,414 3/1958 Bussard. 2,901,652 8/1959 Fridrich 313-1081 2,944,1777/1960 Piper 313-1081 2,945,976 7/1960 Fridrich 313-1081 3,000,7729/1961 Lunn 154-525 3,110,836 11/1963 Blazek et al 313-108.1

EARL M. BERGERT, Primary Examiner.

DOUGLAS J. DRUMMOND, Examiner.

1. IN THE MANUFACTURE OF AN ELECTROLUMINESCENT CELL COMPRISING A LAYEROF ELECTROLUMINESCENT PHOSPHOR SANDWICHED BETWEEN A PAIR OF ELECTRODELAYERS AT LEAST ONE OF WHICH IS LIGHT-TRANSMITTING, THE METHOD WHICHCOMPRISES THE STEPS OF PRELIMINARILY FORMING A SELF-SUPPORTING, TOUGH,FLEXIBLE AND LIGHT-TRANSMITTING ELECTRICALLY CONDUCTIVE SHEET, WHICH CANBE READILY HANDLED AND CUT TO SIZE WITHOUT FRACTURING, BY APPLYING ASHEET OF EXTREMELY FRAGILE ELECTRICALLY CONDUCTIVE GLASS PAPER COMPRISEDOF ELECTRICALLY CONDUCTIVE GLASS FIBERS TO A FACE OF A FLEXIBLETHERMOPLASTIC SHEET AND LAMINATING THE SAID SHEETS TOGETHER UNDERPRESSURE AND HEAT SO THAT THE SAID CONDUCTIVE GLASS PAPER IS PARTLYEMBEDDED IN THE SAID FACE OF THE PLASTIC SHEET BUT SUFFICIENTLY EXPOSEDTHEREAT TO RENDER THE SAID FACE ELECTRICALLY CONDUCTIVE, AND THENAFFIXING TO THE SAID CONDUCTIVE FACE OF THE PLASTIC SHEET A LAYER OFELECTROLUMINESCENT PHOSPHOR AND AN OVERLAYER OF ELECTRICALLY CONDUCTIVEMATERIAL.